For some network system, such as television distribution systems (e.g., cable and satellite), data flows from a service provider to users. User activities have little system-wide impact. For other network systems (e.g., Internet Protocol Television (IPTV) systems), data flows bi-directionally to and from the service provider and the users. User activities may have a large system-wide impact. An understanding of user activities and realistic models of user activity are lacking. Previous models for user activities are sometimes quite different from reality and can potentially lead to incorrect estimation of system performance. For example, while a constant-rate Poisson process is widely used as a workload model for some systems, the constant-rate Poisson process may not accurately model bursty activity. An alternative to a user activity model is to directly use actual trace data for the evaluation of system performance. Such data, even when anonymized, may contain too much commercial information and user information to be publicly distributed.